proc0_post: Fix some locking issues

• Filter out PRS_NEW procs as rufetch() tries taking the thread lock which may not yet be initialized.
• Hold PROC_LOCK to ensure stability of iterating the threads.
• p_rux fields are protected by the process statlock as well.

MFC after: 2 weeks
Reviewed by: kib
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D15809